This invention relates to radiant surface combustors. More specifically, this invention relates to radiant surface combustors in combination with heat transfer surfaces for use in heat engines.
Heat engines in general are well known in the prior art, and typically comprise engines of either the open or closed cycle type wherein heat is transferred to an engine working fluid. For example, in an open cycle heat engine such as a gas turbine, a mixture of fuel and an oxidizer such as air is burned in a combustion chamber, and the burned products of combustion are used to heat an engine working fluid. Specifically, the products of combustion are mixed with excess air to form a heated working fluid which is expanded through an expansion turbine to obtain a work output. In a closed cycle heat engine, such as a steam engine, closed cycle Brayton engine, or the like, a fuel and air mixture is burned in a combustion chamber to generate heat for heating an engine working fluid, such as water or air, by heat exchange through a fixed boundary without intermixture between the combustion products and the working fluid.
A major problem with fuel-burning heat engines comprises the presence of noxious pollutants in the products of combustion. More specifically, combustion of the fuel is normally incomplete whereby polluting exhaust emissions such as unburned hydrocarbons, carbon monoxide, and oxides of nitrogen are present. In the prior art, heat engines have been designed to produce satisfactorily low levels of unburned hydrocarbons and carbon monoxide, but the levels of oxides of nitrogen have remained objectionably high. The presence of nitrogen oxides in the exhaust is largely due to the relatively long flame residence times and high flame temperatures, typically about 4000.degree. R or more, of conventional flame propagation-type combustors.
In some prior art combustion applications, radiant surface combustors have been used in lieu of traditional flame propagation-type combustions, and have exhibited improved exhaust emission characteristics particularly with regard to the presence of oxides of nitrogen. That is, some prior art combustors have been proposed wherein a pressurized gaseous fuel-air mixture is forced through a porous combustor element, and wherein combustion occurs generally at the surface of the combustor element to produce primarily radiant heat energy. See, for example, U.S. Pat. Nos. 1,223,308; 3,027,936; 3,063,493; 3,155,142; 3,179,156; 3,191,659; 3,208,247; 3,217,701; 3,231,202; 3,275,497; 3,383,159; and 3,650,661. However, these prior art applications of radiant surface combustors have generally been limited to space-heater type applications. Radiant surface combustors have not been used with heat engines since they have been generally incapable of providing sufficient quantities of radiant heat energy at a sufficiently high temperature level for satisfactory operation of a heat engine.
The present invention overcomes the problem and disadvantages of the prior art by providing a radiant surface combustor for producing relatively large quantities of radiant heat energy, and for efficiently transferring the heat energy to the working fluid of the engine.